Within the semiconductor industry, a number of prior art methods are available for patterning a resist layer. The following is a description of just one type of method commonly used for patterning a resist layer when using a pulsed radiation source. A substrate is coated with a resist layer and placed on a substrate stage in a lithographic printer. The lithographic printer has a pulsed radiation source and a reticle that is disposed between the radiation source and the resist layer. The reticle has transparent and opaque sections that correspond to the desired pattern to be printed within the resist layer.
The substrate is aligned to the reticle. The aligning procedure is an active process where the printer acquires the position of predetermined alignment marks of the reticle and the substrate, so that the printer can locate any position on the substrate. This aligning process includes methods such as global alignment and enhanced global alignment which are well known in the art. After aligning, the substrate stage is moved to the first area of the resist layer to be patterned. The printer may locally align to the first area to adjust for errors that occur when moving the stage. The substrate stage is positioned, substantially stops moving and is allowed to settle, and the radiation source emits a pulse of radiation. When exposed to the radiation pulse, the transparent sections of the reticle allow a significant amount of the radiation to pass through the reticle, and the opaque sections prevent substantially all of the radiation from reaching the resist layer, thereby patterning the resist layer. The substrate stage moves a predetermined distance to the next area to be patterned, the substrate stage may be locally aligned to the second area, the substrate stage substantially stops moving and is allowed to settle, and another radiation pulse is emitted causing another portion of the resist layer to be patterned. This type of method for patterning the resist layer is sometimes referred to as "step and repeat" or "stepping". The lithographic printer used for the method is called a "stepper", and the area of the resist layer under the reticle during a radiation pulse is a "stepping field". The stepper typically patterns a resist layer that includes a plurality of stepping fields.
The prior art method of patterning a resist layer consumes time that reduces machine throughput and puts stress on precision mechanical parts that control substrate stage position. FIG. 1 is an illustration of the relative movement of the reticle with respect to a substrate 10 using the prior art method. The stepping path 11 is shown by a dashed line with directional arrows. Every time the substrate is substantially stopped in the x-direction or the y-direction, an "x" is marked across the path 11. Each radiation pulse is shown as a ".vertline." marked across and normal to the path. The marks in FIG. 1 that appear to be asterisks are an "x" superimposed over a ".vertline.", meaning that the substrate is substantially stopped relative to the reticle during a radiation pulse. A six-inch diameter wafer typically has about 50 stepping fields. Therefore, the substrate stage may be started and stopped about 50 times per substrate.
In a lithographic printer employing a pulsed radiation source, a plurality of radiation pulses may be needed to pattern a resist layer because each pulse has a significant amount of noise or the radiation dose for a pulse is not enough to substantially pattern the resist layer. Each stepping field is substantially patterned using N radiation pulses, where each radiation pulse has 1/N of the total radiation dose needed to substantially pattern the resist layer. In a prior art process, the first stepping field is substantially patterned with all of its radiation pulses before the substrate is realigned or moved to another stepping field. The total time needed to substantially pattern a stepping field by the prior art method is approximated by the following equation: EQU t.sub.PA =N(t.sub.P +t.sub.R)+t.sub.ST +t.sub.S
where
t.sub.PA is the total time using prior art methods; PA1 N is the number of radiation pulses per stepping field; PA1 t.sub.P is the time of a radiation pulse; and PA1 t.sub.R is the recovery time between two consecutive pulses. PA1 t.sub.ST is time to move between stepping fields; and PA1 t.sub.S is the substrate stage settling time. PA1 s.sub.PA is the total variance using the prior art method; PA1 s.sub.S is the variance of the systematic errors; and PA1 s.sub.R is the variance of the random errors.
In a currently available pulsed x-ray stepper such as one made by Hampshire Instruments, a pulse time is between about 2 ns to 20 ns, the recovery time is tenths of a second, and the step and settle time (combination of the time to move between stepping fields and the substrate stage settle time) is at least a tenth of a second.
The misalignment variance using the prior art method is about: EQU (s.sub.PA).sup.2 =(s.sub.S).sup.2 +(s.sub.R).sup.2
where